This invention relates to the installation of an offshore platform of the type used for oil and gas drilling and/or production operations. More particularly, this invention relates to the mating of a plurality of prefabricated jacket sections during the installation of an offshore platform.
Construction costs for offshore platforms favor onshore construction, with deep water installation providing difficult handling, transportation, launching and upending procedures for placing a prefabricated tower structure at the selected site. Therefore, the use of prefabricated, stackable jacket sections provides an alternative responsive to both onshore construction costs and offshore transportation, handling and installation costs. However, multipiece jacket platform installations require that the respective jacket sections or stages be mated on site. The mating system must guide the jacket sections together to join load-bearing members and to hold the jacket sections in place until the jacket sections are securely interconnected.
In the past, the lowermost jacket section or jacket bottom section is first deployed and secured to the ocean floor. Docking poles and corresponding receptacles carried on respective jacket sections have been used to aid the mating process as succeeding jacket sections are stacked in a vertical assembly. However, the prior art mating process has proceeded with a substantially horizontally coplanar array of substantially vertical outboard docking poles carried on a first jacket member engaging a corresponding substantially horizontally coplanar array of substantially vertical receptacles of a second jacket section as the first and second jacket sections are joined. A successful deployment in this method requires simultaneous alignment of each corresponding pair within these respective arrays at a single touchdown. However, such alignment is difficult to achieve and sustain while approaching touchdown. The effects of wave action on both jacket sections and any surface vessels controlling the operation, shifts in buoyancy during deployment, currents, and normal deflection inherent in deployment of a structure as massive as many jacket sections render total alignment for all points at touchdown a difficult requirement.
Further, minor misalignment can produce disastrous results. The docking poles and receptacles are particularly disposed to damage from misaligned landing, but even major structural components of the jacket sections are jeopardized. Further, correction can be difficult and a failure to fully correct misalignment problems can compromise the structural integrity of the entire offshore platform. Thus, the all-points landing technique risks substantial redeployment and repair costs.
Clearly there is a need for a simpler technique and facilitating apparatus for mating prefabricated multipiece jacket sections in the construction of offshore platforms.